The present invention has particular relation to the apparatus for thickening pulp and paper stock shown in Seifert et al U.S. Pat. No. 4,722,793, issued Feb. 2, 1988 to the assignee of this application. The apparatus disclosed in that patent comprises, as its major component, a pair of liquid-impervious rolls rotatably mounted in spaced relation on substantially horizontal axes. An endless wire is trained around these rolls in wrapping relation with a substantial portion of the surfaces of each thereof, and means are provided for driving one of the rolls to cause this wire to travel around the rolls while cooperating therewith to define a space mounted by the rolls and the opposed upper and lower runs of the wire.
A headbox is mounted in this space and includes an outlet for the pulp suspension to be thickened which is delivered into the space between one of the rolls and the portion of the wire wrapping that roll, whereby this pulp suspension is trapped between the wire and the roll. The rolls are driven at a speed effecting the development of centrifugal force causing liquid to be expressed from between the wire and rolls with the resulting thickening of the pulp carried on the inner surface of the wire, and means are provided to collect and remove this thickened pulp from the space bounded by the wire and rolls.
As is pointed out in the above patent, the apparatus disclosed therein is capable of operating at very much higher speeds than conventional thickening apparatus of the decker type, namely speeds in the range of 1500-4000 feet per minute as compared with decker operation at a linear speed in the range of 200-300 feet per minute. As a result, the capacity of such apparatus, in terms of tons per day of pulp, is correspondingly high, and in developing that apparatus for the marketplace, it was found that special provision should be made for facilitating the collection and removal of the thickened pulp.
Doctor blades of conventional types and mountings, such as are shown in the above patent, were found to have certain disadvantages for this purpose. More specifically, the use of a doctor blade in contact with the surface of the roll was found to be undesirable, for a number of reasons. For one, the resulting friction between the edge of the blade and the roll caused accelerated abrasion damage to both the blade and the surface of the roll, and this abrasion damage was magnified when the apparatus was used, as is conventional, for the thickening of waste paper pulp stock which had been only roughly screened and therefore contained many metal and other hard contaminant particles such as paperclips and staples.
Another problem appeared when a conventionally pressure loaded doctor blade was used in conjunction with the standard practice of utilizing means for reciprocating the blade longitudinally of the roll. It developed in the course of experimental use that over a relatively short period, fibers began to build up on the leading edge of the doctor blade until enough fiber had accumulated to force the blade so far away from the roll as to lose its doctoring effect. It therefore became clear that a new doctoring technique was needed.